Mars rover reaches a milestone

A panorama of Mars’ Marathon Valley, shot by Opportunity

NASA’s Opportunity rover has now been motoring across Mars for more than 5,000 days, far exceeding scientists’ expectations for the golf cart–size “robotic geologist.” The solar-powered machine landed on the Red Planet’s southern hemisphere on Jan. 25, 2004—Sol 1, in NASA speak. The term “sol” is used because on Mars a solar day, the time it takes for a planet to rotate once around its axis, is 40 minutes longer than on Earth. Mars’ southern hemisphere doesn’t get much sunlight during the winter, which lasts nearly twice as long as Earth’s, and NASA expected the rover to be operational for just 90 sols. But technicians at the space agency were able to extend Opportunity’s lifespan by tilting it northward during the winter, maximizing its solar panels’ exposure to the reduced sunlight. During its extended working life, Opportunity has taken more than 225,000 photos, traversed 28 miles, and found evidence that ancient Mars likely had ground and surface water. Its latest discovery is a network of possible “rock stripes” similar to those that can form on mountain slopes on Earth when a mix of silt, sand, and gravel repeatedly freezes and thaws over time. “Five thousand sols after the start of our 90-sol mission,” NASA project manager John Callas tells Space.com, “this amazing rover is still showing us surprises.”